1. Field of the Invention
The invention relates to an image reader. The invention relates especially to an image reader using a fluorescent lamp in which a dielectric barrier discharge is used.
2. Description of Related Art
Conventionally, various lamps are used as the light source of an image reader. When using a halogen lamp as the light source, there are the disadvantages that at least 80% of the power consumed is converted into heat and that, due to emission by means of a luminous filament, there is no resistance to vibrations. In a light source using a fluorescent lamp of the hot cathode type, in which mercury vapor is used, the thermal efficiency is higher than in a halogen lamp. However, there is the disadvantage that, depending on the distribution of the mercury vapor, the start-up characteristic of the lamp, the amount of light and the spectral distribution are affected. When using a fluorescent lamp of the cold cathode type, in which mercury vapor is used, there is the same disadvantage as in a fluorescent lamp of the hot cathode type because mercury vapor is used.
In view of these disadvantages and with respect to low power consumption, good start-up characteristic and little influence of the environment, a fluorescent lamp using a rare gas, such as xenon or the like, is used. Furthermore, a fluorescent lamp of the outer electrode type is used in which there is no electrode in the discharge vessel and which is operated by a dielectric barrier discharge to obtain a long service life.
FIG. 5 shows a schematic of one such image reader 1. A manuscript P is placed on the document glass 10. By illuminating the manuscript with the light emitted from a fluorescent lamp 2 (hereinafter also called only a “lamp”), the reflected light is incident on a CCD (charge-coupled device) line sensor 4 (hereinafter also called only a sensor 4). The fluorescent lamp 2 together with the inverter circuit 3 forms a lighting part which is located in a unit 5 which is triggered parallel to the document glass 10 based on a scanning signal S1 from the controller 6, as shown in the drawings. The sensor 4 extends in front of the manuscript page and can recognize the image part of the manuscript P which corresponds to the direction of extension of the sensor.
The sequence of operation of such an image reader 1 is described below.
First, the unit 5 starts scanning based on the signal S1 from the controller 6. When a signal S2 is sent from the controller 6 to the sensor 4, the image in the sensor 4 which had been received until then is reset and it is switched into the state in which a new image can be received. The signal S2 is also sent at the same time to the inverter circuit 3. The image received by the sensor 4 is sent as a signal S3 to the controller 6 in which processing of such image coupling or the like takes place. The control element 6, in this way, controls the images from the sensor 4 which were subjected to partial recognition.
The timing of the start of emission of the fluorescent lamp 2 is synchronized with the timing of switching of the image received by the sensor 4. The reason for this is that the sensor 4, as was described above, resets the received image at a specified time, for example, every 150 microseconds to 300 microseconds, and pulse emission of the fluorescent lamp 2 must be prevented at the instant of reset.
FIGS. 6(a) and 6(b) each schematically show this switching of the image received by the CCD line sensor 4 and the timing of the pulse emission of the fluorescent lamp 2. FIG. 6(a) shows the timing of the switching of the image received by the sensor 4, and FIG. 6(b) shows the timing of pulse emission of the lamp 2.
The drawings show that the sensor 4 erases the information of the image received until the controller 6 sends a signal S2 to the sensor 4 at time t1. The sensor 4 then remains on stand-by in the state in which it can recognize the image to be received next. Reference number T12 labels the period up to time t2 in which the next signal S2 is sent. This period T12 is the period in which the same image is received.
On the other hand, if the controller 6 sends the signal S2 to the inverter circuit 3, the inverter circuit 3 starts a drive, this signal S2 acting as the trigger. Then, based on an oscillator located in the inverter circuit, switching is performed, by which the lamp is subjected to pulsed luminous operation with a given interval.
By this operation, the lamp 2 undergoes pulse emission for a period T12 in which the sensor 4 receives a divided image, with a certain frequency, for example, 20 times (in the case of one read period of 300 microseconds and an oscillation period within the inverter of 15 microseconds). When the next signal S2 is sent to the inverter circuit 3 (time t2), the inverter circuit again starts a drive when the signal is received, synchronously with switching of the image received by the sensor.
The reason why this synchronization is necessary is the following:
In the case in which pulse emission of the lamp 2 is delayed for any reason, this delay time accumulates according to the frequency of pulse emission. This results in the phenomenon that the timing of the pulse emission of the lamp 2 agrees with the timing of the reset of the sensor 4 or that a given pulse frequency is not obtained in an image recognition interval and that for example there is no pulse. This phenomenon means that the entire emission amount of the lamp fluctuates in one read period. This results in the problem that the sensor can no longer clearly and accurately recognize images. To eliminate these disadvantages, each time the image received by the sensor 4 is switched, the timing of the emission of the lamp 2 is re-synchronized.
Also, when the timing of the start of pulse emission of the fluorescent lamp is synchronized in this way with the timing of the switching of the image received by the sensor, however in the period in which the image is received (T12, T23 in FIGS. 6(a), 6(b) and the like) in the timing of the pulse emission, a deviation is formed by which the same disadvantages as the above described disadvantages occur. Specifically, the last pulse emission agrees with the timing of image switching of the sensor and that, furthermore, the last pulse emission does not take place.
FIGS. 7(a), 7(b) and 7(c) each show a timing chart of one such state. FIG. 7(a) shows the timing of the switching of the image received by the sensor 4, as in FIG. 6(a). FIGS. 7(b) and 7(c) show the timing of the pulse emission of a lamp 2, as in FIG. 6(b). Here, there are feasibly 20 pulse emissions in one period.
FIG. 7(b) shows that, for the period T12, the 20-time emission P120 remains within the period, while for the period T23, the 20-time emission P220 agrees with the timing t3 of switching of the image recognized by the sensor. Furthermore, it is shown in FIG. 7(c) that the 20-time pulse emission P220 does not take place in the period T23 in which actually one emission is to take place.
When this situation arises, in the case of FIG. 7(b) a wrong signal is received by the sensor because the timing of the pulse emission agrees with the timing of sensor image switching. In case 7(c) only an amount of light can be obtained which corresponds to 19-time emission, although actually 20-times the amount of light should be received.
Furthermore, besides the two specific examples described above, it can of course also happen that within one period the timing of the pulse emission is accelerated and an amount of light is obtained which is larger than the amount of light which is actually desired (for the specific examples shown in FIG. 7, there are 21 pulse emissions within one period).
The occurrence of such a situation causes a change in the amount of light and mingling of anomalous signals in the image reading activity. As a result, a clear image cannot be obtained.
Especially recently has there been a tendency to increase the scanning rate of the unit due to the demand for an increase in the image reading rate. The emission frequency of the lamp in the time in which a divided image is received (T12, T23, T34 in FIGS. 6(a), 6(b) and FIGS. 7(a), 7(b) and 7(c)) is therefore reduced. Thus, there is the tendency for the luminous quantities to increase per pulse to maintain the total amount of light in one receiving period, instead of the preceding situation.
This circumstance is described specifically below.
When the frequency of the pulse emission at a receiving time for a divided image of roughly 150 microseconds to 300 microseconds decreases, for example, from 20 time to 15 times, the luminous quantities per time of the lamp must be increased, i.e., an emission is required by which the same total amount of light can be obtained by 15-times emission in order to maintain the entire amount of light. When the read rate is increased, the probability is increased that the above described deviation and the above described absence of pulse emission occur. At the same time, the ratio of the amount of emission of one pulse to the total amount of emission in one read period is increased. The disadvantages of the deviation and of the absence of a pulse therefore become more serious.